Optical wavelength converting device and process for producing the same

ABSTRACT

A photosensitive resist layer is formed on one surface of a single-polarized ferroelectric substance having nonlinear optical effects. The resist layer has properties such that, when light is irradiated to the resist layer, only exposed areas of the resist layer or only unexposed areas of the resist layer become soluble in a developing solvent. The resist layer is then exposed to near-field light in a periodic pattern with a device, which receives exposure light and produces the near-field light in the periodic pattern. The resist layer is then developed to form a periodic pattern. A periodic electrode is then formed on the one surface of the ferroelectric substance by utilizing the periodic pattern of the resist layer as a mask, the periodic electrode being formed at positions corresponding to opening areas of the mask. An electric field is applied across the ferroelectric substance by utilizing the periodic electrode to set regions of the ferroelectric substance, which stand facing the periodic electrode, as domain inversion regions.

This is a divisional application of Ser. No. 09/649,013 filed Aug. 28,2000, now U.S. Pat. No. 6,998,223. The entire disclosure of priorapplication Ser. No. 09/649,013 is considered part of the disclosure ofthe accompanying divisional application and is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical wavelength converting device forconverting a fundamental wave into its second harmonic, or the like.This invention particularly relates to an optical wavelength convertingdevice having a periodic domain inversion structure. This invention alsorelates to a process for producing the optical wavelength convertingdevice. This invention further relates to a solid laser for converting aproduced laser beam into its second harmonic by the utilization of theoptical wavelength converting device and radiating out the secondharmonic.

2. Description of the Related Art

A technique, wherein a fundamental wave is converted into its secondharmonic by the utilization of an optical wavelength converting devicehaving a periodic domain inversion structure has been proposed byBleombergen, et al. in Phys. Rev., Vol. 127, No. 6, 1918 (1962). Theperiodic domain inversion structure is provided with regions, in whichspontaneous polarization (domain) of a ferroelectric substance havingnonlinear optical effects is inverted periodically. With the proposedtechnique, phase matching between a fundamental wave and its secondharmonic can be effected by setting such that a period Λ of the domaininversion regions may be integral multiples of a coherence length Λc,which may be represented by Formula (1) shown below.Λc=2π/{β(2ω)−2β(ω)}  (1)in which β(2ω) represents the propagation constant of the secondharmonic, and β(ω) represents the propagation constant of thefundamental wave.

In cases where wavelength conversion is performed by using a bulkcrystal of a nonlinear optical material, the wavelength at which thephase matching is effected is limited to a specific wavelength that isinherent to the crystal. However, with the proposed technique, the phasematching can be effected efficiently by selecting the period Λ of thedomain inversion regions, which period satisfies Formula (1), withrespect to an arbitrary wavelength.

One of techniques for forming the periodic domain inversion structuredescribed above has been proposed in, for example, Japanese UnexaminedPatent Publication No. 7(1995)-72521. With the proposed technique forforming the periodic domain inversion structure, after a periodicelectrode in a predetermined pattern is formed on one surface of asingle-polarized ferroelectric substance having nonlinear opticaleffects, an electric field is applied through corona charge across theferroelectric substance by the utilization of the periodic electrode anda corona wire, which is located on the surface side of the ferroelectricsubstance opposite to the one surface of the ferroelectric substance,and regions of the ferroelectric substance which stand facing theperiodic electrode are thereby set as local area limited domaininversion regions.

A different technique for forming the periodic domain inversionstructure described above has been proposed in, for example, JapaneseUnexamined Patent Publication No. 4(1992)-335620. With the proposedtechnique for forming the periodic domain inversion structure, anentire-area electrode is formed on a surface of a ferroelectricsubstance on the side opposite to a surface on which a periodicelectrode in a predetermined pattern is formed, an electric field isapplied across the ferroelectric substance by the utilization of theentire-area electrode and the periodic electrode, and local area limiteddomain inversion regions are thereby formed.

As a technique for forming the periodic electrode, a technique, whereinridge regions having predetermined shapes in a predetermined pattern areformed on one surface of a ferroelectric substance, and electrodefingers of a periodic electrode are formed on the surfaces of the ridgeregions, has been proposed in, for example, Japanese Unexamined PatentPublication No. 10(1998)-170966.

In cases where the periodic domain inversion structure is formed by theutilization of the periodic electrode in the manner described above,particularly as for a Z-cut ferroelectric substance plate, there is astrong possibility that, as the period of the periodic electrode is setto be short in order for a second harmonic, or the like, having a shortwavelength to be generated, domain inversion regions, which are adjacentto each other and extend through the ferroelectric substance from theareas corresponding to electrode fingers of the periodic electrode, willbecome connected with each other.

The problems described above will be described hereinbelow withreference to FIG. 7. FIG. 7 is a graph showing results of evaluation ofperiodicity of various bulk-form periodic domain inversion structures,each of which is formed in LiNbO₃ doped with MgO (hereinbelow referredto simply as MgO-LN) by the utilization of a periodic electrode havingan electrode line width (i.e., the line width of each of the electrodefingers of the periodic electrode) A, the evaluation being made withrespect to various different values of a period Λ of domain inversionregions and various different values of a duty ratio D (D=A/Λ). In FIG.7, the “◯” mark indicates that the periodicity is good over a length ofat least 1 mm. The “Δ” mark indicates that the periodicity is good onlyover a length of less than 1 mm or that the regions in which theperiodicity is good occur sporadically. The “x” mark indicates that fewregions in which the periodicity is good occur.

As shown in FIG. 7, in order for good periodicity of the periodic domaininversion structure to be obtained, it is efficient to set the dutyratio D at a small value, i.e., to set the electrode line width A at asmall value. Also, in cases where the period Λ of the domain inversionregions is at most 7 μm, it is necessary for the duty ratio D to be setat a value of at most 0.15. In cases where the domain inversion lengthis approximately 1 mm, the duty ratio D should thus be set at a value ofat most 0.15. In the cases of large areas (in cases where the domaininversion length is approximately 3 mm to 4 mm), such that the inversionperiodicity may be enhanced, the duty ratio D should be set at a valuesmaller than the value of at most 0.15.

In cases where the periodic domain inversion structure is formed by theutilization of the periodic electrode, each of the domain inversionregions is formed over a region slightly wider than the regioncorresponding to the electrode line width A due to the spread of theelectric field. Therefore, even if the duty ratio D is set at a valuesmaller than 0.15, the periodic domain inversion structure can beformed, in which the ratio between the width of each domain inversionregion and the width of each non-inversion region is approximately equalto 1:1.

In view of the above circumstances, in cases where a second harmonic, orthe like, having a short wavelength falling within, for example, theblue region or the ultraviolet region is to be generated, it isnecessary for a periodic electrode having a markedly small electrodeline width A to be formed. However, heretofore, it was difficult to forma periodic electrode having a markedly small electrode line width A.Particularly, with respect to the optical wavelength converting devicein which the periodic domain inversion structure is formed in the bulkform in a crystal of a Z-cut plate of MgO-LN, an example in which asecond harmonic having a wavelength falling within the wavelength regionof at most 470 nm has not heretofore been reported. The term “periodicdomain inversion structure in a bulk form in a crystal of a Z-cut plate”as used herein means the periodic domain inversion structure in whichthe domain inversion regions are formed over a range extending from aposition in the vicinity of a +Z surface of the plate to a position inthe vicinity of a −Z surface of the plate.

In cases where a second harmonic having a wavelength falling within thewavelength region of at most 470 nm is to be generated with theaforesaid type of the optical wavelength converting device, if theelectrode line width A of the periodic electrode employed for theformation of the periodic domain inversion structure is set at a valueof at most 0.3 μm, a periodic domain inversion structure reliably havinggood periodicity over a wide area can be formed.

As techniques for forming a periodic electrode having a small electrodeline width A, an EB drawing technique, an FIB deposition technique, andthe like, have heretofore been known. However, the conventionaltechniques for forming a periodic electrode having a small electrodeline width A are not appropriate for large-area patterning and have alow throughput and a productivity markedly lower than the level ofproductivity required for mass production.

As a technique capable of coping with large-area patterning, a techniqueutilizing a contraction exposure apparatus has heretofore been known.However, the technique utilizing the contraction exposure apparatus hasthe draw-backs in that the cost of the contraction exposure apparatus ismarkedly high and it is difficult to obtain an electrode line width Ashorter than the wavelength of exposure light.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a process forproducing an optical wavelength converting device, wherein a periodicelectrode having a markedly small electrode line width is capable ofbeing formed, and a bulk-form periodic domain inversion structure, inwhich domain inversion regions are formed with a markedly short periodthat has heretofore been impossible, is thereby capable of being formed.

Another object of the present invention is to provide an opticalwavelength converting device having a bulk-form periodic domaininversion structure, in which domain inversion regions are formed with amarkedly short period that has heretofore been impossible.

A further object of the present invention is to provide a solid laser,wherein the optical wavelength converting device is utilized, a producedlaser beam is capable of being converted into its second harmonic havinga markedly short wavelength, and the second harmonic is radiated outfrom the solid laser.

The present invention provides a first process for producing an opticalwavelength converting device having a periodic domain inversionstructure, in which a periodic electrode is formed on one surface of asingle-polarized ferroelectric substance having nonlinear opticaleffects, and an electric field is applied across the ferroelectricsubstance by the utilization of the periodic electrode in order to setregions of the ferroelectric substance, which stand facing the periodicelectrode, as local area limited domain inversion regions, the processcomprising the steps of:

-   -   i) forming a photosensitive resist layer on the one surface of        the ferroelectric substance, the resist layer having properties        such that, when light is irradiated to the resist layer, only        exposed areas of the resist layer or only unexposed areas of the        resist layer become soluble in a developing solvent,    -   ii) exposing the resist layer to near-field light in a periodic        pattern with means, which receives exposure light and produces        the near-field light in the periodic pattern,    -   iii) developing the resist layer, which has been exposed to the        near-field light, to form a periodic pattern in the resist        layer, and    -   iv) forming the periodic electrode on the one surface of the        ferroelectric substance by utilizing the periodic pattern of the        resist layer as a mask, the periodic electrode being formed at        positions corresponding to opening areas of the mask.

The present invention also provides a second process for producing anoptical wavelength converting device having a periodic domain inversionstructure, in which a periodic electrode is formed on one surface of asingle-polarized ferroelectric substance having nonlinear opticaleffects, and an electric field is applied across the ferroelectricsubstance by the utilization of the periodic electrode in order to setregions of the ferroelectric substance, which stand facing the periodicelectrode, as local area limited domain inversion regions, the processcomprising the steps of:

-   -   i) forming an electrode material layer on the one surface of the        ferroelectric substance,    -   ii) forming a photosensitive resist layer on the electrode        material layer, the resist layer having properties such that,        when light is irradiated to the resist layer, only exposed areas        of the resist layer or only unexposed areas of the resist layer        become soluble in a developing solvent,    -   iii) exposing the resist layer to near-field light in a periodic        pattern with means, which receives exposure light and produces        the near-field light in the periodic pattern,    -   iv) developing the resist layer, which has been exposed to the        near-field light, to form a periodic pattern in the resist        layer, and    -   v) etching the electrode material layer by utilizing the        periodic pattern of the resist layer as an etching mask, such        that portions of the electrode material layer at positions        corresponding to opening areas of the mask are removed by the        etching, whereby the periodic electrode is formed.

The present invention further provides a third process for producing anoptical wavelength converting device having a periodic domain inversionstructure, in which a periodic electrode is formed on one surface of asingle-polarized ferroelectric substance having nonlinear opticaleffects, and an electric field is applied across the ferroelectricsubstance by the utilization of the periodic electrode in order to setregions of the ferroelectric substance, which stand facing the periodicelectrode, as local area limited domain inversion regions, the processcomprising the steps of:

-   -   i) forming a first resist layer and a second resist layer in        this order on the one surface of the ferroelectric substance,        the first resist layer being removable by etching, the second        resist layer being photosensitive and having properties such        that, when light is irradiated to the second resist layer, only        exposed areas of the second resist layer or only unexposed areas        of the second resist layer become soluble in a developing        solvent,    -   ii) exposing the second resist layer to near-field light in a        periodic pattern with means, which receives exposure light and        produces the near-field light in the periodic pattern,    -   iii) developing the second resist layer, which has been exposed        to the near-field light, to form a periodic pattern in the        second resist layer,    -   iv) etching the first resist layer by utilizing the periodic        pattern of the second resist layer as an etching mask to form a        periodic pattern composed of the first resist layer and the        second resist layer, and    -   v) forming the periodic electrode on the one surface of the        ferroelectric substance by utilizing the periodic pattern, which        is composed of the first resist layer and the second resist        layer, as a mask, the periodic electrode being formed at        positions corresponding to opening areas of the mask.

The present invention still further provides a fourth process forproducing an optical wavelength converting device having a periodicdomain inversion structure, in which a periodic electrode is formed onone surface of a single-polarized ferroelectric substance havingnonlinear optical effects, and an electric field is applied across theferroelectric substance by the utilization of the periodic electrode inorder to set regions of the ferroelectric substance, which stand facingthe periodic electrode, as local area limited domain inversion regions,the process comprising the steps of:

-   -   i) forming an electrode material layer on the one surface of the        ferroelectric substance,    -   ii) forming a first resist layer and a second resist layer in        this order on the electrode material layer, the first resist        layer being removable by etching, the second resist layer being        photosensitive and having properties such that, when light is        irradiated to the second resist layer, only exposed areas of the        second resist layer or only unexposed areas of the second resist        layer become soluble in a developing solvent,    -   iii) exposing the second resist layer to near-field light in a        periodic pattern with means, which receives exposure light and        produces the near-field light in the periodic pattern,    -   iv) developing the second resist layer, which has been exposed        to the near-field light, to form a periodic pattern in the        second resist layer,    -   v) etching the first resist layer by utilizing the periodic        pattern of the second resist layer as an etching mask to form a        periodic pattern composed of the first resist layer and the        second resist layer, and    -   vi) etching the electrode material layer by utilizing the        periodic pattern, which is composed of the first resist layer        and the second resist layer, as an etching mask, such that        portions of the electrode material layer at positions        corresponding to opening areas of the mask are removed by the        etching, whereby the periodic electrode is formed.

In the third and fourth processes for producing an optical wavelengthconverting device in accordance with the present invention, the secondresist layer should preferably have a film thickness of at most 100 nm.Also, the third and fourth processes for producing an optical wavelengthconverting device in accordance with the present invention shouldpreferably be modified such that the first resist layer is formed from anon-photosensitive material, and the etching performed for the firstresist layer is dry etching.

In the first, second, third and fourth processes for producing anoptical wavelength converting device in accordance with the presentinvention, the exposure light should preferably have a wavelengthfalling within the range of 250 nm to 450 nm.

Also, the first, second, third and fourth processes for producing anoptical wavelength converting device in accordance with the presentinvention should preferably be modified such that the means, whichreceives the exposure light and produces the near-field light in theperiodic pattern, is a mask comprising a light-transmitting member,which is capable of transmitting the exposure light, and a metalpattern, which has opening areas and is formed on the light-transmittingmember, the near-field light being radiated out from the metal pattern,and

the mask comprising the light-transmitting member and the metal patternis located such that the metal pattern is in close contact with theresist layer, which is laid bare on the ferroelectric substance, or themetal pattern is located close to the resist layer, which is laid bareon the ferroelectric substance, such that the near-field light reachesthe resist layer, which is laid bare on the ferroelectric substance, theexposure light being irradiated to the mask comprising thelight-transmitting member and the metal pattern in this state.

Further, the first, second, third and fourth processes for producing anoptical wavelength converting device in accordance with the presentinvention should preferably be modified such that the means, whichreceives the exposure light and produces the near-field light in theperiodic pattern, is an optical stamp constituted of alight-transmitting member, which is capable of transmitting the exposurelight and has a concavity-convexity pattern formed on one surface, theoptical stamp operating such that, when the exposure light is guidedfrom within the light-transmitting member to the one surface of thelight-transmitting member and is caused to undergo total reflection, thenear-field light in a pattern in accordance with the concavity-convexitypattern formed on the one surface of the light-transmitting member isradiated out, and

the optical stamp is located such that the one surface of the opticalstamp provided with the concavity-convexity pattern is in close contactwith the resist layer, which is laid bare on the ferroelectricsubstance, or the one surface of the optical stamp provided with theconcavity-convexity pattern is located close to the resist layer, whichis laid bare on the ferroelectric substance, such that the near-fieldlight reaches the resist layer, which is laid bare on the ferroelectricsubstance, the exposure light being irradiated to the optical stamp inthis state.

Furthermore, the first, second, third and fourth processes for producingan optical wavelength converting device in accordance with the presentinvention should preferably be modified such that the means, whichreceives the exposure light and produces the near-field light in theperiodic pattern, is a probe provided with an opening having a diametershorter than a wavelength of the exposure light, the probe being causedto scan on the resist layer, which is laid bare on the ferroelectricsubstance, the exposure light being irradiated to the probe in thisstate.

Also, in the first, second, third and fourth processes for producing anoptical wavelength converting device in accordance with the presentinvention, the ferroelectric substance should preferably be LiNbO₃ dopedwith MgO (MgO-LN). In such cases, the periodic electrode shouldpreferably have an electrode line width of at most 0.3 μm.

The present invention also provides a first optical wavelengthconverting device, comprising a crystal of a Z-cut plate of LiNbO₃ dopedwith MgO, domain inversion regions being formed periodically in a bulkform in the crystal,

wherein the domain inversion regions are formed with a period fallingwithin the range of 1.0 μm to 4.6 μm.

The present invention further provides a second optical wavelengthconverting device, comprising a crystal of a Z-cut plate of LiNbO₃ dopedwith MgO, domain inversion regions being formed periodically in a bulkform in the crystal,

wherein the optical wavelength converting device is constituted toradiate out a wavelength-converted wave having a wavelength fallingwithin the range of 320 nm to 470 nm.

The present invention still further provides a third optical wavelengthconverting device, comprising a crystal of a Z-cut plate of LiNbO₃ dopedwith MgO, domain inversion regions being formed periodically in a bulkform in the crystal,

wherein the domain inversion regions are formed with a period fallingwithin the range of 1.0 μm to 4.6 μm, and

the optical wavelength converting device is constituted such that, whena fundamental wave having a wavelength falling within the range of 640nm to 940 nm impinges upon the optical wavelength converting device, theoptical wavelength converting device radiates out a second harmonichaving a wavelength falling within the range of 320 nm to 470 nm withthe period of the domain inversion regions acting as a first-orderperiod for pseudo-phase matching.

The present invention also provides a solid laser, comprising the first,second, or third optical wavelength converting device in accordance withthe present invention, the solid laser being constituted to convert aproduced laser beam into its second harmonic and to radiate out thesecond harmonic.

With the processes for producing an optical wavelength converting devicein accordance with the present invention, the photosensitive resist isexposed to the near-field light, which oozes from the periodic patternhaving a line width shorter than the wavelength of the exposure light,and the exposed resist is then developed. Therefore, a periodicelectrode having an electrode line width of at most 100 nm, i.e., aperiod of at most 200 nm, can be formed. Thus a periodic electrodehaving a short electrode line width, which was impossible withconventional lithography, can be obtained.

Specifically, in cases where the periodic electrode is formed on the onesurface of the ferroelectric substance by utilizing the periodic patternof the resist layer as a mask, the periodic electrode being formed atthe positions corresponding to the opening areas of the mask, the linewidth of each of the opening areas of the mask may be set at a value ofat most 100 nm.

In cases where the electrode material layer is formed on the one surfaceof the ferroelectric substance, the electrode material layer is etchedby utilizing the periodic pattern of the resist layer as the etchingmask, such that the portions of the electrode material layer at thepositions corresponding to the opening areas of the mask are removed bythe etching, and the periodic electrode is thereby formed, the linewidth of each of the areas other than the opening areas of the mask(i.e., the line width of each of the areas remaining as the resistlayer) may be set at a value of at most 100 nm.

With the third and fourth processes for producing an optical wavelengthconverting device in accordance with the present invention, wherein thedouble-layered resist comprising the first resist layer and the secondresist layer is employed, in cases where the ferroelectric substance hasa step-like area and an area to which the near-field light cannot reachif only one resist layer is formed occurs, the first resist layer canact to form a flat surface and, therefore, the film thickness of thesecond resist, which is photosensitive and is formed on the first resistlayer, can be uniformized. Accordingly, the near-field light can beradiated out uniformly even in a large-area pattern, and a fine patternof the second resist layer, which is photosensitive, can be formed. Thefirst resist layer is then patterned with a conventional etchingtechnique by utilizing the pattern of the photosensitive second resistlayer as the mask. In this manner, a fine pattern can be formed easilyand at a low cost.

With the processes for producing an optical wavelength converting devicein accordance with the present invention, wherein the mask provided withthe metal pattern or the optical stamp provided with theconcavity-convexity pattern is employed as the means, which receives theexposure light and produces the near-field light in the periodicpattern, the advantages over the scanning exposure can be obtained inthat the exposure of a large-area periodic pattern can be performedinstantaneously and, therefore, the optical wavelength converting devicecan be produced with a high throughput and at a low cost.

With the processes for producing an optical wavelength converting devicein accordance with the present invention, wherein the periodic electrodehaving a markedly small line width is capable of being formed in themanner described above, the optical wavelength converting devicecomprising a crystal of a Z-cut plate of LiNbO₃ doped with MgO, in whichthe domain inversion regions are formed periodically in a bulk form inthe crystal, can be obtained, wherein the domain inversion regions areformed with a period falling within the range of 1.0 μm to 4.6 μm, andwherein the optical wavelength converting device is constituted suchthat, when a fundamental wave having a wavelength falling within therange of 640 nm to 940 nm impinges upon the optical wavelengthconverting device, the optical wavelength converting device radiates outa second harmonic having a wavelength falling within the range of 320 nmto 470 nm with the period of the domain inversion regions acting as thefirst-order period for the pseudo-phase matching.

As the optical wavelength converting device comprising a crystal of aZ-cut plate of LiNbO₃ doped with MgO, in which the domain inversionregions are formed periodically in a bulk form in the crystal, anoptical wavelength converting device capable of radiating out a secondharmonic having a wavelength of at most 470 nm has not heretofore beenfurnished. Since the absorption end of MgO-LN is 320 nm, it ispractically impossible to radiate a second harmonic having a wavelengthshorter than 320 nm from the optical wavelength converting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1G are schematic views showing steps in a first embodimentof the process for producing an optical wavelength converting device inaccordance with the present invention,

FIG. 2 is a schematic view showing a final step in the first embodimentof the process for producing an optical wavelength converting device inaccordance with the present invention,

FIG. 3 is a side view showing a solid laser, in which an opticalwavelength converting device obtained with the first embodiment of theprocess for producing an optical wavelength converting device inaccordance with the present invention is employed,

FIGS. 4A to 4F are schematic views showing steps in a second embodimentof the process for producing an optical wavelength converting device inaccordance with the present invention,

FIG. 5 is a schematic view showing a step in a third embodiment of theprocess for producing an optical wavelength converting device inaccordance with the present invention,

FIG. 6 is a schematic view showing a step in a fourth embodiment of theprocess for producing an optical wavelength converting device inaccordance with the present invention, and

FIG. 7 is a graph showing results of evaluation of periodicity ofvarious bulk-form periodic domain inversion structures, each of which isformed in a ferroelectric substance by the utilization of a periodicelectrode having an electrode line width A, the evaluation being madewith respect to various different values of a period Λ of domaininversion regions and various different values of a duty ratio D(D=A/Λ).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will hereinbelow be described in further detailwith reference to the accompanying drawings.

FIGS. 1A to 1G and FIG. 2 show steps of producing an optical wavelengthconverting device in a first embodiment of the process for producing anoptical wavelength converting device in accordance with the presentinvention. FIGS. 1A to 1G show steps of forming a periodic electrode.FIG. 2 shows a step of inverting a spontaneous polarization (domain) ofa ferroelectric substance by utilizing the periodic electrode havingbeen formed with the steps shown in FIGS. 1A to 1G.

How the periodic electrode is formed will be described hereinbelow withreference to FIGS. 1A to 1G. In this embodiment, MgO-LN is employed asthe ferroelectric substance having nonlinear optical effects. Firstly,as illustrated in FIG. 1A, a Z-cut MgO-LN plate 10 is prepared. TheMgO-LN plate 10 is subjected to single polarization, and the two Zsurfaces of the MgO-LN plate 10 are subjected to mirror polishing. Thethickness of the MgO-LN plate 10 is thus set at 0.3 mm.

Thereafter, as illustrated in FIG. 1B, a resist layer 11 constituted ofa photosensitive material is formed with a spin coating technique or aspraying technique on one surface (a +Z surface) 10 a of the MgO-LNplate 10. The thickness of the resist layer 11 is set at a valueapproximately equal to or smaller than the oozing depth of near-fieldlight, which oozing depth is ordinarily approximately equal to 50 nm.

Thereafter, as illustrated in FIG. 1C, a mask 12 for generating thenear-field light in a periodic pattern is located such that the mask 12is in close contact with the resist layer 11. The mask 12 comprises amask substrate, which is constituted of a dielectric material, such asglass, and a lattice-like metal pattern, which has fine opening areas 12a, 12 a, . . . and is formed on the mask substrate. In this embodiment,as will be clear from the explanation made later, each of the openingareas 12 a, 12 a, . . . of the metal pattern corresponds to one ofelectrode fingers of the periodic electrode to be formed, and each ofmetal areas 12 b, 12 b, . . . corresponds to one of spaces betweenadjacent electrode fingers.

The mask 12 is located such that the opening areas 12 a, 12 a, . . . ofthe metal pattern stand side-by-side with respect to a X axis directionof the MgO-LN plate 10. Also, a period A of the metal pattern of themask 12 is set at a value of 2.1 μm so as to act as the first-orderperiod with respect to a wavelength of 380 nm of a second harmonic,which will be described later.

As illustrated in FIG. 1D, exposure light L, such as i-rays (having awavelength of 365 nm), is then irradiated from the rear side of the mask12 (i.e., from the upper side in FIG. 1D) to the mask 12. As a result,near-field light Ln oozes from the opening areas 12 a, 12 a, . . . ofthe metal pattern, and the resist layer 11 is exposed to the near-fieldlight Ln.

Thereafter, the resist layer 11 is developed with a developing solution,and the portions of the resist layer 11, which were exposed to thenear-field light Ln, become soluble in a developing solvent. In thismanner, as illustrated in FIG. 1E, a positive type of periodic pattern11 a of the resist layer 11 is formed. Thereafter, as illustrated inFIG. 1F, the periodic pattern 11 a is utilized as a mask, and chromium(Cr) 13 acting as an electrode material is deposited to a thickness of,for example, 20 nm by vacuum evaporation. As a result, Cr 13 isdeposited on a one surface 10 a of the MgO-LN plate 10 and only atpositions corresponding to opening areas of the periodic pattern 11 a ofthe resist layer 11. In lieu of Cr 13 being deposited by vacuumevaporation, tantalum (Ta) may be deposited by a sputtering technique,or the like.

Thereafter, as illustrated in FIG. 1G, the positive type of periodicpattern 11 a of the resist layer 11 is removed by a lift-off technique,and a periodic electrode 13 a having a period Λ of 2.1 μm is therebyformed on the one surface 10 a of the MgO-LN plate 10. Since the mask 12was located as described above such that the opening areas 12 a, 12 a, .. . of the metal pattern stand side-by-side with respect to the X axisdirection of the MgO-LN plate 10, the electrode fingers constituting theperiodic electrode 13 a stand side-by-side with respect to the X axisdirection of the MgO-LN plate 10.

In this embodiment, the width of each of the opening areas 12 a, 12 a, .. . of the metal pattern is set at a value of 0.2 μm and, therefore, anelectrode line width A of the periodic electrode 13 a is set at a valueof 0.2 μm. Accordingly, in this case, a duty ratio D (D=A/Λ) of theperiodic electrode 13 a is equal to 0.1. The value of the duty ratio Dis lower than the value of 0.15 described above.

How the spontaneous polarization (domain) of the MgO-LN plate 10 isinverted by the utilization of the periodic electrode 13 a will bedescribed hereinbelow with reference to FIG. 2. As illustrated in FIG.2, the MgO-LN plate 10 is located on an electrically conductive jig 1such that the periodic electrode 13 a is in contact with theelectrically conductive jig 1. The electrically conductive jig 1 isformed from an electrically conductive material, such as copper orstainless steel, and is grounded through a grounding wire 2.

Also, a corona wire 3 is located above a −Z surface 10 b of the MgO-LNplate 10, and a high voltage electric source 4 is connected to thecorona wire 3. In this state, an electric field is applied throughcorona charge across the MgO-LN plate 10 by the utilization of thecorona wire 3 and the high voltage electric source 4. At this time, thetemperature of the MgO-LN plate 10 is set at 100° C., and the distancebetween the corona wire 3 and the MgO-LN plate 10 is set at 10 mm. Inthis state, an electric voltage of 5 kV is applied for one second fromthe high voltage electric source 4 via the corona wire 3. After theelectric field has been applied, the periodic electrode 13 a is removedfrom the MgO-LN plate 10.

A test was made for confirming the formation of domain inversion regionsin the MgO-LN plate 10. In the test, the Y surface of the MgO-LN plate10 was cut and polished. Thereafter, selective etching was performed byuse of a mixed etching solution containing hydrofluoric acid and nitricacid. When the cross-section (the Y surface) of the MgO-LN plate 10 wasobserved, it was confirmed that periodic domain inversion regions wereformed at positions corresponding to the positions of the electrodefingers of periodic electrode 13 a and with the predetermined periodcorresponding to the period of the periodic electrode 13 a. It was alsoconfirmed that each of the periodic domain inversion regions was formeduniformly to extend from the −Z surface to the +Z surface and haduniform shape in the Y surface.

An optical wavelength converting device constituted of the MgO-LN plate10 having been obtained in the manner described above will be describedhereinbelow with reference to FIG. 3. In the manner described above,periodic domain inversion regions 21, 21, . . . are formed, which standside-by-side with respect to the X axis direction of the MgO-LN plate10. Thereafter, the +X surface and the −X surface of the MgO-LN plate 10are polished. Non-reflection coating layers are then formed on the +Xsurface and the −X surface of the MgO-LN plate 10, and light passagesurfaces 20 a and 20 b are thereby formed. In this manner, a bulkcrystal type of optical wavelength converting device 20 shown in FIG. 3is obtained.

As illustrated in FIG. 3, the bulk crystal type of optical wavelengthconverting device 20 having the periodic domain inversion structure islocated on an output side of an Ar laser pumped titanium sapphire laser22. A laser beam 23 is produced by the Ar laser pumped titanium sapphirelaser 22, converged by a converging lens 24, and caused to impinge uponthe bulk crystal type of optical wavelength converting device 20. Inthis case, such that phase matching may be effected with respect to thefundamental wave having a wavelength of 760 nm and the second harmonichaving a wavelength of 380 nm, with dispersion due to variation of therefractive index of the MgO-LN for different wavelengths being takeninto consideration, the period Λ of the periodic domain inversionregions 21, 21, . . . (which period is equal to the period of theperiodic electrode 13 a) is set at a value of 2.1 μm.

The Ar laser pumped titanium sapphire laser 22 produces the laser beam23 having a wavelength of 760 nm as the fundamental wave. The outputpower of the Ar laser pumped titanium sapphire laser 22 is 400 mW. Thelaser beam 23 impinges upon the bulk crystal type of optical wavelengthconverting device 20 and is converted into a second harmonic 25 having awavelength of 380 nm, which is one-half of the wavelength of the laserbeam 23. The second harmonic 25 undergoes phase matching (i.e., thepseudo-phase matching) in the periodic domain inversion regions. Asdescribed above, the periodic domain inversion regions 21, 21, . . .have good periodicity. Therefore, the phase matching is effectedappropriately, and the second harmonic 25 with power of 0.5 mW isobtained.

Steps in a second embodiment of the process for producing an opticalwavelength converting device in accordance with the present inventionwill be described hereinbelow with reference to FIGS. 4A to 4F. In FIGS.4A to 4F, similar elements are numbered with the same reference numeralswith respect to FIGS. 1A to 1G.

Firstly, as illustrated in FIG. 4A, the MgO-LN plate 10, which is of thesame type as that employed in the first embodiment described above, isprepared. A Cr layer 30 having a thickness of 20 nm, which acts as anelectrode material layer, a first resist layer 31 constituted of anorganic high-molecular weight material, and a second resist layer 32constituted of a photosensitive material are formed in this order on theone surface (+Z surface) 10 a of the MgO-LN plate 10 and with a spincoating technique or a spraying technique. The first resist layer 31 andthe second resist layer 32 constitute a double-layered resist 33.

Thereafter, as illustrated in FIG. 4B, the mask 12, which is of the sametype as that employed in the first embodiment described above, islocated such that the mask 12 is in close contact with thedouble-layered resist 33. As in the first embodiment described above,the mask 12 having the metal areas 12 b, 12 b, . . . and the openingareas 12 a, 12 a, . . . of the metal pattern are located such that theopening areas 12 a, 12 a, . . . stand side-by-side with respect to the Xaxis direction of the MgO-LN plate 10. Also, as in the first embodimentdescribed above, as will be clear from the explanation made later, eachof the opening areas 12 a, 12 a, . . . of the metal pattern correspondsto one of electrode fingers of the periodic electrode to be formed, andeach of metal areas 12 b, 12 b, . . . corresponds to one of spacesbetween adjacent electrode fingers.

As illustrated in FIG. 4C, the exposure light L, such as i-rays (havinga wavelength of 365 nm), is then irradiated from the rear side of themask 12 (i.e., from the upper side in FIG. 4C) to the mask 12. As aresult, the near-field light Ln oozes from the opening areas 12 a, 12 a,. . . of the metal pattern, and the second resist layer 32 is exposed tothe near-field light Ln.

Thereafter, the second resist layer 32 is developed with a developingsolution, and the portions of the second resist layer 32, which wereexposed to the near-field light Ln, become soluble in a developingsolvent. In this manner, as illustrated in FIG. 4D, a negative type ofperiodic pattern of the second resist layer 32 is formed. Thereafter, asillustrated in FIG. 4E, the periodic pattern of the second resist layer32 is utilized as an etching mask, and the first resist layer 31 and theCr layer 30 are subjected to dry etching with an O₂ plasma.

Thereafter, as illustrated in FIG. 4F, the second resist layer 32 andthe first resist layer 31 are removed, and a periodic electrode 30 aconstituted of Cr is thereby formed on the one surface 10 a of theMgO-LN plate 10. Since the mask 12 was located as described above suchthat the opening areas 12 a, 12 a, . . . of the metal pattern standside-by-side with respect to the X axis direction of the MgO-LN plate10, the electrode fingers constituting the periodic electrode 30 a standside-by-side with respect to the X axis direction of the MgO-LN plate10.

The quality of the first resist layer 31 does not deteriorate due toexposure to light. Therefore, when the first resist layer 31 isdissolved, the first resist layer 31 and the second resist layer 32 canbe removed easily. Alternatively, the first resist layer 31 and thesecond resist layer 32 may be peeled off with plasma ashing.

The photosensitive resist constituting the second resist layer 32 may bea positive type of resist having properties such that, when the resistis exposed to light, only the exposed portions of the resist becomessoluble in a developing solution. Also, the thickness of the secondresist layer 32 should preferably be approximately identical with theoozing depth of the near-field light or shorter than the oozing depth ofthe near-field light.

Basically, as the organic high-molecular weight material constitutingthe first resist layer 31, one of various materials, which is capable ofbeing etched with the O₂ plasma, may be employed.

In the manner described above, the periodic electrode 30 a is formed onthe one surface 10 a of the MgO-LN plate 10. Thereafter, the spontaneouspolarization (domain) of the MgO-LN plate 10 can be inverted by theutilization of the periodic electrode 30 a. The domain inversionprocessing may be performed by using, for example, the apparatus shownin FIG. 2.

The first resist layer 31 and the second resist layer 32 willhereinbelow be described in more detail.

The first resist layer 31 is formed from a material capable ofundergoing dry etching, particularly an organic high-molecular weightmaterial. The first resist layer 31 should preferably be formed from amaterial, which does not form an intermediated mixed layer with thesecond resist layer 32 overlaid on the first resist layer 31. Therefore,the first resist layer 31 should preferably be formed from an organichigh-molecular weight material, which does not dissolve in the solventemployed in the second resist layer 32. Alternatively, the first resistlayer 31 should preferably be formed from an organic high-molecularweight material, which dissolves at normal temperatures in the solventemployed in the second resist layer 32, and which is capable of beingconverted with processing, such as heating, into a crosslinked networkstructure that substantially forms no intermediated mixed layer with thesecond resist layer 32.

As a technique for utilizing an organic high-molecular weight material,which is capable of being converted with processing, such as heating,into a crosslinked network structure, a technique may be employed,wherein a layer of a resist for i-rays or a resist for g-rays, whichcontains a novolak resin and a naphthoquinone diazide compound and isutilized for production of semi-conductor devices, or the like, isapplied to a necessary film thickness and is thereafter cured with heattreatment. Alternatively, a technique may be employed, wherein a layerof a negative type of resist, which contains an alkali-soluble resin,such as a novolak resin or a polyhydroxystyrene, an acid cross-linkingagent, and a photo acid generating agent, is applied and is thereaftercured with entire-surface exposure to light. As another alternative, atechnique may be employed, wherein a layer of a negative type of resist,which contains an alkali-soluble resin, such as a novolak resin or apolyhydroxystyrene, a polyfunctional monomer, and a photo-polymerizationinitiating agent or a thermal polymerization initiating agent, isapplied and is thereafter cured with entire-surface exposure to light orwith heat treatment.

The first resist layer 31 may also contain various additives for variouspurposes, such as furalene and its derivatives.

The second resist layer 32 is formed from a photosensitive resistmaterial having properties such that, when the resist material isexposed to the near-field light, only the exposed portions of the resistmaterial or only the unexposed portions of the resist material becomesoluble in a developing solvent, and the other portions of the resistmaterial have dry etching resistance. The resist material shouldpreferably be a material, which contains a compound having silicon atomsand in which the proportion of silicon in the solid content is equal toat least a predetermined value. In cases where the dry etching isperformed with an oxygen-containing plasma, from the view point ofoxygen plasma resistance, the proportion of silicon in the solid contentin the resist material should preferably be comparatively high. However,ordinarily, if the proportion of silicon is markedly high, the patternforming characteristics, edge roughness of the pattern or residues, andthe like, will become bad. Therefore, the proportion of silicon in thesolid content in the resist material should preferably be at least 1%,should more preferably fall within the range of 4% to 50%, and shouldmost preferably fall within the range of 5% to 30%.

Examples of the resist materials, which may be employed for the secondresist layer 32, include the resist materials described in U.S. Pat.Nos. 5,338,818, 5,422,223, 5,866,306, 5,385,804, 5,399,462, 5,238,773,4,481,049, 4,689,289 and 4,822,716; and EP No. 229629A1. Further, theresist materials shown in the following formula, which are disclosed inJapanese Unexamined Patent Publication No. 7(1995)-114188, can beemployed for the second resist layer 32.

The photosensitive resin composition contains polysilane having astructure represented by the formula (in which each of R₁–R₄ isindependently selected from the group consisting of optionallysubstituted aliphatic, alicyclic and aromatic hydrocarbon groups, andeach of m and n is an integer), an optical radial generating agent andan oxidizing agent. Further, the resist materials shown in the followingformulae, which are disclosed in Japanese Unexamined Patent PublicationNo. 11(1999)-20224, can be employed for the second resist layer 32.

The positive-type silicone-containing photosensitive compositioncomprising:

(a) a polymer which has a repeat unit expressed with the followinggeneral formula (I) and/or (II), and which is water-insoluble andalkali-soluble;

(b) a compound which generates an acid when subject to an activity beamof light or radiation; and

(c) a polymer which has in the side chain thereof a repeat unitexpressed with the following general formula (II), the general formula(IV), or the general formula (V), and which exhibits properties that thesolubility in an alkali developer increases due to the action of anacid.

(X in formulae (I) and (II) is a group selected from the groupconsisting of —C(═O)—R group, —CH(OH)—R group, and a carboxyl group, anda plurality of X groups in the formula may be the same or different. Rshows a hydrocarbon group which may have a hydrogen atom or a substitutetherein. R′–R′″″ may be the same or different, and is selected from thegroup consisting of an alkyl group, a cycloalkyl machine, an alkoxygroup, an alkenyl group, an aralkyl group and a phenyl group all ofwhich may have a hydroxyl group and/or a substituent. Y is an alkylgroup, an alkoxy group or a siloxyl machine. R₀ represents a groupselected from the group consisting of an aliphatic hydrocarbon group andan aromatic hydrocarbon group which may have a hydrogen atom, a halogenatom and/or a substituent. Each of l, m, n and q is 0 or a positiveinteger, and p is a positive integer.)

(Each of Ra, Rb and Rc in formulae (III)–(V) is independently ahydrocarbon group which may have a hydrogen atom and/or a substituent. sis an integer equal to 2 or greater.)

Among the above-enumerated resist materials for the second resist layer32, materials capable of being developed with an aqueous alkalideveloping solution are preferable for the capability of forming a goodpattern with high developing power such that no organic waste liquidoccurs and little swelling occurs. Specifically, pattern formingmaterials, which contain a water-insoluble, aqueous alkali-soluble,silicon-containing polymer and a photosensitive compound, arepreferable.

More specifically, the following pattern forming materials arepreferable: pattern forming materials, which contain a water-insoluble,aqueous alkali-soluble, silicone-containing polymer and a naphthoquinonediazide compound and/or a diazo ketone compound; positive types ofpattern forming materials, which contain a water-insoluble, aqueousalkali-soluble, silicone-containing polymer, a compound capable ofgenerating an acid with exposure to active light rays or radiation, anda high- or low-molecular weight compound having a group decomposablewith an acid and having properties such that the solubility in anaqueous alkali developing solution increases by the action of an acid;negative types of pattern forming materials, which contain a functionalgroup-containing, water-insoluble, silicone-containing polymer having agroup decomposable with an acid and having properties such that thesolubility in an aqueous alkali developing solution increases by theaction of an acid, a compound capable of generating an acid withexposure to active light rays or radiation, and a high- or low-molecularweight compound having a group crosslinkable with an acid and havingproperties such that the solubility in an aqueous alkali developingsolution decreases by the action of an acid; negative types of patternforming materials, which contain a water-insoluble, silicone-containingpolymer having an olefinically unsaturated group and having propertiessuch that the solubility in an aqueous alkali developing solutiondecreases through a polymerization reaction, and a compound capable ofgenerating polymerization reaction initiating ability with exposure toactive light rays or radiation; and negative types of pattern formingmaterials, which contain a water-insoluble, aqueous alkali-soluble,silicone-containing polymer, a compound capable of generatingpolymerization reaction initiating ability with exposure to active lightrays or radiation, and a high- or low-molecular weight compound havingan olefinically unsaturated group and having properties such that thesolubility in an alkali developing solution decreases through apolymerization reaction.

Among the above-enumerated pattern forming materials, the patternforming materials, which contain a water-insoluble, aqueousalkali-soluble, silicone-containing polymer, a compound capable ofgenerating an acid with exposure to active light rays or radiation, anda high- or low-molecular weight compound having a group decomposablewith an acid and having properties such that the solubility in anaqueous alkali developing solution increases by the action of an acid,are particularly preferable. Such pattern forming materials aredescribed in detail in, for example, Japanese Patent Application No.10(1998)-354878 with reference to the general formula, the explanationof the general formula, and examples. In the second embodiment of theprocess for producing an optical wavelength converting device inaccordance with the present invention, such types of the pattern formingmaterials can be employed appropriately. Also, various additives capableof being added to the pattern forming materials are described in detailin Japanese Patent Application No. 10(1998)-354878. The additives canalso be employed appropriately in the second embodiment of the processfor producing an optical wavelength converting device in accordance withthe present invention.

A third embodiment of the process for producing an optical wavelengthconverting device in accordance with the present invention will bedescribed hereinbelow with reference to FIG. 5. In the third embodiment,an optical stamp 40 is employed. The optical stamp 40 is constituted ofa light-transmitting member, which is capable of transmitting theexposure light L and has a concavity-convexity pattern formed on onesurface (the lower surface in FIG. 5), and the near-field light iscapable of being radiated out from the concavity-convexity pattern. Asillustrated in FIG. 5, the optical stamp 40 is located such that the onesurface provided with the concavity-convexity pattern is in closecontact with the resist layer 11. When the exposure light L isintroduced into the optical stamp 40 and caused to undergo totalreflection from the one surface of the optical stamp 40, the near-fieldlight Ln is radiated out from the convex areas of theconcavity-convexity pattern. In this manner, the resist layer 11 can beexposed to the near-field light Ln.

In the third embodiment, after the resist layer 11 has been exposed tothe near-field light Ln, the development of the resist, the formation ofthe electrode, and the domain inversion processing may be performed, forexample, in the same manner as that in the first embodiment describedabove. The optical stamp 40 has the advantages in that, since a metal isnot used as in the aforesaid mask, the optical stamp 40 can be obtainedat a low cost.

A fourth embodiment of the process for producing an optical wavelengthconverting device in accordance with the present invention will bedescribed hereinbelow with reference to FIG. 6. In the fourthembodiment, scanning exposure utilizing a probe 50 is performed. Theprobe 50 is provided with an opening having a diameter shorter than thewavelength of the exposure light and radiates out the near-field lightLn. The probe 50 is driven by scanning drive means (not shown) to scanin a periodic pattern mode on the resist layer 11. In this manner, theresist layer 11 is exposed in the periodic pattern to the near-fieldlight Ln.

In the fourth embodiment, after the resist layer 11 has been exposed tothe near-field light Ln, the development of the resist, the formation ofthe electrode, and the domain inversion processing may be performed, forexample, in the same manner as that in the first embodiment describedabove.

The exposure system employed in the third embodiment or the fourthembodiment described above can also be employed in cases where thedouble-layered resist is employed as in the second embodiment describedabove.

In addition, all of the contents of Japanese Patent Application Nos.11(1999)-241062 and 11(1999)-293802 are incorporated into thisspecification by reference.

1. An optical wavelength converting device having a periodic domaininversion structure, in which a periodic electrode is formed on onesurface of a single-polarized ferroelectric substance having nonlinearoptical effects, and an electric field is applied across theferroelectric substance by the utilization of the periodic electrode inorder to set regions of the ferroelectric substance, which stand facingthe periodic electrode, as local area limited domain inversion regions,the optical wavelength converting device produced by a processcomprising the steps of: i) forming a first resist layer and a secondresist layer in this order on the one surface of the ferroelectricsubstance, the first resist layer being removable by etching, the secondresist layer being photosensitive and having properties such that, whenlight is irradiated to the second resist layer, only exposed areas ofthe second resist layer or only unexposed areas of the second resistlayer become soluble in a developing solvent, ii) exposing the secondresist layer to near-field light in a periodic pattern with means, whichreceives exposure light and produces the near-field light in theperiodic pattern, iii) developing the second resist layer, which hasbeen exposed to the near-field light, to form a periodic pattern in thesecond resist layer, iv) etching the first resist layer by utilizing theperiodic pattern of the second resist layer as an etching mask to form aperiodic pattern composed of the first resist layer and the secondresist layer, and v) forming the periodic electrode on the one surfaceof the ferroelectric substance by utilizing the periodic pattern, whichis composed of the first resist layer and the second resist layer, as amask, the periodic electrode being formed at positions corresponding toopening areas of the mask.
 2. A solid laser, comprising the opticalwavelength converting device as defined in claim 1, the solid laserbeing constituted to convert a produced laser beam into its secondharmonic and to radiate out the second harmonic.
 3. An opticalwavelength converting device having a periodic domain inversionstructure, in which a periodic electrode is formed on one surface of asingle-polarized ferroelectric substance having nonlinear opticaleffects, and an electric field is applied across the ferroelectricsubstance by the utilization of the periodic electrode in order to setregions of the ferroelectric substance, which stand facing the periodicelectrode, as local area limited domain inversion regions, the opticalwavelength converting device produced by a process comprising the stepsof: i) forming an electrode material layer on the one surface of theferroelectric substance, ii) forming a first resist layer and a secondresist layer in this order on the electrode material layer, the firstresist layer being removable by etching, the second resist layer beingphotosensitive and having properties such that, when light is irradiatedto the second resist layer, only exposed areas of the second resistlayer or only unexposed areas of the second resist layer become solublein a developing solvent, iii) exposing the second resist layer tonear-field light in a periodic pattern with means, which receivesexposure light and produces the near-field light in the periodicpattern, iv) developing the second resist layer, which has been exposedto the near-field light, to form a periodic pattern in the second resistlayer, v) etching the first resist layer by utilizing the periodicpattern of the second resist layer as an etching mask to form a periodicpattern composed of the first resist layer and the second resist layer,and vi) etching the electrode material layer by utilizing the periodicpattern, which is composed of the first resist layer and the secondresist layer, as an etching mask, such that portions of the electrodematerial layer at positions corresponding to opening areas of the maskare removed by the etching, whereby the periodic electrode is formed. 4.A solid laser, comprising the optical wavelength converting device asdefined in claim 3, the solid laser being constituted to convert aproduced laser beam into its second harmonic and to radiate out thesecond harmonic.
 5. An optical wavelength converting device, having aperiodic domain inversion structure, in which a periodic electrode isformed on one surface of a single-polarized ferroelectric substancehaving nonlinear optical effects, and an electric field is appliedacross the ferroelectric substance by the utilization of the periodicelectrode in order to set regions of the ferroelectric substance, whichstand facing the periodic electrode, as local area limited domaininversion regions, the optical wavelength converting device produced bya process comprising the steps of: i) forming a first resist layer and asecond resist layer in this order on the one surface of theferroelectric substance, the first resist layer being removable byetching, the second resist layer being photosensitive and havingproperties such that, when light is irradiated to the second resistlayer, only exposed areas of the second resist layer or only unexposedareas of the second resist layer become soluble in a developing solvent,ii) exposing the second resist layer to near-field light in a periodicpattern with means, which receives exposure light and produces thenear-field light in the periodic pattern, iii) developing the secondresist layer, which has been exposed to the near-field light, to form aperiodic pattern in the second resist layer, iv) etching the firstresist layer by utilizing the periodic pattern of the second resistlayer as an etching mask to form a periodic pattern composed of thefirst resist layer and the second resist layer, and v) forming theperiodic electrode on the one surface of the ferroelectric substance byutilizing the periodic pattern, which is composed of the first resistlayer and the second resist layer, as a mask, the periodic electrodebeing formed at positions corresponding to opening areas of the mask;wherein the optical wavelength converting device comprises a crystal ofa Z-cut plate of LiNbO₃ doped with MgO, domain inversion regions beingformed periodically in a bulk form in the crystal, wherein the domaininversion regions are formed with a period falling within the range of1.0 μm to 4.6 μm.
 6. An optical wavelength converting device having aperiodic domain inversion structure, in which a periodic electrode isformed on one surface of a single-polarized ferroelectric substancehaving nonlinear optical effects, and an electric field is appliedacross the ferroelectric substance by the utilization of the periodicelectrode in order to set regions of the ferroelectric substance, whichstand facing the periodic electrode, as local area limited domaininversion regions, the optical wavelength converting device produced bya process comprising the steps of: i) forming a first resist layer and asecond resist layer in this order on the one surface of theferroelectric substance, the first resist layer being removable byetching, the second resist layer being photosensitive and havingproperties such that, when light is irradiated to the second resistlayer, only exposed areas of the second resist layer or only unexposedareas of the second resist layer become soluble in a developing solvent,ii) exposing the second resist layer to near-field light in a periodicpattern with means, which receives exposure light and produces thenear-field light in the periodic pattern, iii) developing the secondresist layer, which has been exposed to the near-field light, to form aperiodic pattern in the second resist layer, iv) etching the firstresist layer by utilizing the periodic pattern of the second resistlayer as an etching mask to form a periodic pattern composed of thefirst resist layer and the second resist layer, and v) forming theperiodic electrode on the one surface of the ferroelectric substance byutilizing the periodic pattern, which is composed of the first resistlayer and the second resist layer, as a mask, the periodic electrodebeing formed at positions corresponding to opening areas of the mask;wherein the optical wavelength converting device comprises a crystal ofa Z-cut plate of LiNbO₃ doped with MgO, domain inversion regions beingformed periodically in a bulk form in the crystal; and wherein theoptical wavelength converting device is constituted to radiate out awavelength-converted wave having a wavelength falling within the rangeof 320 nm to 470 nm.
 7. An optical wavelength converting device having aperiodic domain inversion structure, in which a periodic electrode isformed on one surface of a single-polarized ferroelectric substancehaving nonlinear optical effects, and an electric field is appliedacross the ferroelectric substance by the utilization of the periodicelectrode in order to set regions of the ferroelectric substance, whichstand facing the periodic electrode, as local area limited domaininversion regions, the optical wavelength converting device produced bya process comprising the steps of: i) forming a first resist layer and asecond resist layer in this order on the one surface of theferroelectric substance, the first resist layer being removable byetching, the second resist layer being photosensitive and havingproperties such that, when light is irradiated to the second resistlayer, only exposed areas of the second resist layer or only unexposedareas of the second resist layer become soluble in a developing solvent,ii) exposing the second resist layer to near-field light in a periodicpattern with means, which receives exposure light and produces thenear-field light in the periodic pattern, iii) developing the secondresist layer, which has been exposed to the near-field light, to form aperiodic pattern in the second resist layer, iv) etching the firstresist layer by utilizing the periodic pattern of the second resistlayer as an etching mask to form a periodic pattern composed of thefirst resist layer and the second resist layer, and v) forming theperiodic electrode on the one surface of the ferroelectric substance byutilizing the periodic pattern, which is composed of the first resistlayer and the second resist layer, as a mask, the periodic electrodebeing formed at positions corresponding to opening areas of the mask;wherein the optical wavelength converting device comprises a crystal ofa Z-cut plate of LiNbO₃ doped with MgO, domain inversion regions beingformed periodically in a bulk form in the crystal; wherein the domaininversion regions are formed with a period falling within the range of1.0 μm to 4.6 μm; and wherein the optical wavelength converting deviceis constituted such that, when a fundamental wave having a wavelengthfalling within the range of 640 nm to 940 nm impinges upon the opticalwavelength converting device, the optical wavelength converting deviceradiates out a second harmonic having a wavelength falling within therange of 320 nm to 470 nm with the period of the domain inversionregions acting as a first-order period for pseudo-phase matching.
 8. Asolid laser, comprising an optical wavelength converting device having aperiodic domain inversion structure, in which a periodic electrode isformed on one surface of a single-polarized ferroelectric substancehaving nonlinear optical effects, and an electric field is appliedacross the ferroelectric substance by the utilization of the periodicelectrode in order to set regions of the ferroelectric substance, whichstand facing the periodic electrode, as local area limited domaininversion regions, the optical wavelength converting device produced bya process comprising the steps of: i) forming a first resist layer and asecond resist layer in this order on the one surface of theferroelectric substance, the first resist layer being removable byetching, the second resist layer being photosensitive and havingproperties such that, when light is irradiated to the second resistlayer, only exposed areas of the second resist layer or only unexposedareas of the second resist layer become soluble in a developing solvent,ii) exposing the second resist layer to near-field light in a periodicpattern with means, which receives exposure light and produces thenear-field light in the periodic pattern, iii) developing the secondresist layer, which has been exposed to the near-field light, to form aperiodic pattern in the second resist layer, iv) etching the firstresist layer by utilizing the periodic pattern of the second resistlayer as an etching mask to form a periodic pattern composed of thefirst resist layer and the second resist layer, and v) forming theperiodic electrode on the one surface of the ferroelectric substance byutilizing the periodic pattern, which is composed of the first resistlayer and the second resist layer, as a mask, the periodic electrodebeing formed at positions corresponding to opening areas of the mask;wherein the optical wavelength converting device comprises a crystal ofa Z-cut plate of LiNbO₃ doped with MgO, domain inversion regions beingformed periodically in a bulk form in the crystal, the solid laser beingconstituted to convert a produced laser beam into its second harmonicand to radiate out the second harmonic; and wherein the domain inversionregions are formed with a period falling within the range of 1.0 μm to4.6 μm.
 9. A solid laser, comprising an optical wavelength convertingdevice having a periodic domain inversion structure, in which a periodicelectrode is formed on one surface of a single-polarized ferroelectricsubstance having nonlinear optical effects, and an electric field isapplied across the ferroelectric substance by the utilization of theperiodic electrode in order to set regions of the ferroelectricsubstance, which stand facing the periodic electrode, as local arealimited domain inversion regions, the optical wavelength convertingdevice produced by a process comprising the steps of: i) forming a firstresist layer and a second resist layer in this order on the one surfaceof the ferroelectric substance, the first resist layer being removableby etching, the second resist layer being photosensitive and havingproperties such that, when light is irradiated to the second resistlayer, only exposed areas of the second resist layer or only unexposedareas of the second resist layer become soluble in a developing solvent,ii) exposing the second resist layer to near-field light in a periodicpattern with means, which receives exposure light and produces thenear-field light in the periodic pattern, iii) developing the secondresist layer, which has been exposed to the near-field light, to form aperiodic pattern in the second resist layer, iv) etching the firstresist layer by utilizing the periodic pattern of the second resistlayer as an etching mask to form a periodic pattern composed of thefirst resist layer and the second resist layer, and v) forming theperiodic electrode on the one surface of the ferroelectric substance byutilizing the periodic pattern, which is composed of the first resistlayer and the second resist layer, as a mask, the periodic electrodebeing formed at positions corresponding to opening areas of the mask;wherein the optical wavelength converting device comprises comprising acrystal of a Z-cut plate of LiNbO₃ doped with MgO, domain inversionregions being formed periodically in a bulk form in the crystal, thesolid laser being constituted to convert a produced laser beam into itssecond harmonic and to radiate out the second harmonic; and wherein theoptical wavelength converting device is constituted to radiate out awavelength-converted wave having a wavelength falling within the rangeof 320 nm to 470 nm.
 10. A solid laser, comprising an optical wavelengthconverting device having a periodic domain inversion structure, in whicha periodic electrode is formed on one surface of a single-polarizedferroelectric substance having nonlinear optical effects, and anelectric field is applied across the ferroelectric substance by theutilization of the periodic electrode in order to set regions of theferroelectric substance, which stand facing the periodic electrode, aslocal area limited domain inversion regions, the optical wavelengthconverting device produced by a process comprising the steps of: i)forming a first resist layer and a second resist layer in this order onthe one surface of the ferroelectric substance, the first resist layerbeing removable by etching, the second resist layer being photosensitiveand having properties such that, when light is irradiated to the secondresist layer, only exposed areas of the second resist layer or onlyunexposed areas of the second resist layer become soluble in adeveloping solvent, ii) exposing the second resist layer to near-fieldlight in a periodic pattern with means, which receives exposure lightand produces the near-field light in the periodic pattern, iii)developing the second resist layer, which has been exposed to thenear-field light, to form a periodic pattern in the second resist layer,iv) etching the first resist layer by utilizing the periodic pattern ofthe second resist layer as an etching mask to form a periodic patterncomposed of the first resist layer and the second resist layer, and v)forming the periodic electrode on the one surface of the ferroelectricsubstance by utilizing the periodic pattern, which is composed of thefirst resist layer and the second resist layer, as a mask, the periodicelectrode being formed at positions corresponding to opening areas ofthe mask; wherein the optical wavelength converting device comprises acrystal of a Z-cut plate of LiNbO₃ doped with MgO, domain inversionregions being formed periodically in a bulk form in the crystal, thesolid laser being constituted to convert a produced laser beam into itssecond harmonic and to radiate out the second harmonic; wherein thedomain inversion regions are formed with a period falling within therange of 1.0 μm to 4.6 μm; and wherein the optical wavelength convertingdevice is constituted such that, when a fundamental wave having awavelength falling within the range of 640 nm to 940 nm impinges uponthe optical wavelength converting device, the optical wavelengthconverting device radiates out the second harmonic having a wavelengthfalling within the range of 320 nm to 470 nm with the period of thedomain inversion regions acting as a first-order period for pseudo-phasematching.
 11. An optical wavelength converting device having a periodicdomain inversion structure, in which a periodic electrode is formed onone surface of a single-polarized ferroelectric substance havingnonlinear optical effects, and an electric field is applied across theferroelectric substance by the utilization of the periodic electrode inorder to set regions of the ferroelectric substance, which stand facingthe periodic electrode, as local area limited domain inversion regions,the optical wavelength converting device produced by a processcomprising the steps of: i) forming an electrode material layer on theone surface of the ferroelectric substance, ii) forming a first resistlayer and a second resist layer in this order on the electrode materiallayer, the first resist layer being removable by etching, the secondresist layer being photosensitive and having properties such that, whenlight is irradiated to the second resist layer, only exposed areas ofthe second resist layer or only unexposed areas of the second resistlayer become soluble in a developing solvent, iii) exposing the secondresist layer to near-field light in a periodic pattern with means, whichreceives exposure light and produces the near-field light in theperiodic pattern, iv) developing the second resist layer, which has beenexposed to the near-field light, to form a periodic pattern in thesecond resist layer, v) etching the first resist layer by utilizing theperiodic pattern of the second resist layer as an etching mask to form aperiodic pattern composed of the first resist layer and the secondresist layer, and vi) etching the electrode material layer by utilizingthe periodic pattern, which is composed of the first resist layer andthe second resist layer, as an etching mask, such that portions of theelectrode material layer at positions corresponding to opening areas ofthe mask are removed by the etching, whereby the periodic electrode isformed; wherein the optical wavelength converting device comprises acrystal of a Z-cut plate of LiNbO₃ doped with MgO, domain inversionregions being formed periodically in a bulk form in the crystal; andwherein the domain inversion regions are formed with a period fallingwithin the range of 1.0 μm to 4.6 μm.
 12. An optical wavelengthconverting device having a periodic domain inversion structure, in whicha periodic electrode is formed on one surface of a single-polarizedferroelectric substance having nonlinear optical effects, and anelectric field is applied across the ferroelectric substance by theutilization of the periodic electrode in order to set regions of theferroelectric substance, which stand facing the periodic electrode, aslocal area limited domain inversion regions, the optical wavelengthconverting device produced by a process comprising the steps of: i)forming an electrode material layer on the one surface of theferroelectric substance, ii) forming a first resist layer and a secondresist layer in this order on the electrode material layer, the firstresist layer being removable by etching, the second resist layer beingphotosensitive and having properties such that, when light is irradiatedto the second resist layer, only exposed areas of the second resistlayer or only unexposed areas of the second resist layer become solublein a developing solvent, iii) exposing the second resist layer tonear-field light in a periodic pattern with means, which receivesexposure light and produces the near-field light in the periodicpattern, iv) developing the second resist layer, which has been exposedto the near-field light, to form a periodic pattern in the second resistlayer, v) etching the first resist layer by utilizing the periodicpattern of the second resist layer as an etching mask to form a periodicpattern composed of the first resist layer and the second resist layer,and vi) etching the electrode material layer by utilizing the periodicpattern, which is composed of the first resist layer and the secondresist layer, as an etching mask, such that portions of the electrodematerial layer at positions corresponding to opening areas of the maskare removed by the etching, whereby the periodic electrode is formed;wherein the optical wavelength converting device comprises a crystal ofa Z-cut plate of LiNbO₃ doped with MgO, domain inversion regions beingformed periodically in a bulk form in the crystal; and wherein theoptical wavelength converting device is constituted to radiate out awavelength-converted wave having a wavelength falling within the rangeof 320 nm to 470 nm
 13. An optical wavelength converting device having aperiodic domain inversion structure, in which a periodic electrode isformed on one surface of a single-polarized ferroelectric substancehaving nonlinear optical effects, and an electric field is appliedacross the ferroelectric substance by the utilization of the periodicelectrode in order to set regions of the ferroelectric substance, whichstand facing the periodic electrode, as local area limited domaininversion regions, the optical wavelength converting device produced bya process comprising the steps of: i) forming an electrode materiallayer on the one surface of the ferro electric substance, ii) forming afirst resist layer and a second resist layer in this order on theelectrode material layer, the first resist layer being removable byetching, the second resist layer being photosensitive and havingproperties such that, when light is irradiated to the second resistlayer, only exposed areas of the second resist layer or only unexposedareas of the second resist layer become soluble in a developing solvent,iii) exposing the second resist layer to near-field light in a periodicpattern with means, which receives exposure light and produces thenear-field light in the periodic pattern, iv) developing the secondresist layer, which has been exposed to the near-field light, to form aperiodic pattern in the second resist layer, v) etching the first resistlayer by utilizing the periodic pattern of the second resist layer as anetching mask to form a periodic pattern composed of the first resistlayer and the second resist layer, and vi) etching the electrodematerial layer by utilizing the periodic pattern, which is composed ofthe first resist layer and the second resist layer, as an etching mask,such that portions of the electrode material layer at positionscorresponding to opening areas of the mask are removed by the etching,whereby the periodic electrode is formed; wherein the optical wavelengthconverting device comprises a crystal of a Z-cut plate of LiNbO₃ dopedwith MgO, domain inversion regions being formed periodically in a bulkform in the crystal; wherein the domain inversion regions are formedwith a period falling within the range of 1.0 μm to 4.6 μm; and whereinthe optical wavelength converting device is constituted such that, whena fundamental wave having a wavelength falling within the range of 640nm to 940 nm impinges upon the optical wavelength converting device, theoptical wavelength converting device radiates out a second harmonichaving a wavelength falling within the range of 320 nm to 470 nm withthe period of the domain inversion regions acting as a first-orderperiod for pseudo-phase matching.